libration

Libration is any of several periodic rocking motions of an orbiting body. The various
librations of the Moon enable, over time, about 59% of the Moon's surface
to be seen from Earth.

Libration in latitude,
a north-south nodding, is caused by the tilt of the Moon's rotation axis
relative to its orbital plane. The average up/down latitude wobbling is
5.13°, corresponding to the Moon's orbital inclination with respect
to the ecliptic, though perturbations by the Sun can add a further +/- 0.9°.

Libration in longitude arises from the difference
between the Moon's varying orbital velocity and its constant rotation rate.
Without any other factors it would average 6.29°, but the Sun's contribution
pushes the peak oscillation in longitude to 7.75°.

Diurnal
libration is an optical rather than a physical libration, amounting
to less than 1°, and stems from the fact that, because of Earth's rotation,
we view the Moon from different angles at moonrise and moonset.

Libration happens when an orbiting body is locked in a synchronous
rotation; Mercury shows it, as do some
other moons in the Solar System.